Many high speed receivers require variable intermediate frequency (IF) and/or radio frequency (RF) gain control in order to optimize dynamic range or other performance parameters. The dynamic range of a radio receiver is the range of input RF signal levels over which the receiver can operate. The low end of the range is governed by the receiver's noise sensitivity while the high end of the dynamic range of the receiver is governed by the receiver's overload or strong signal handling performance. In the particular case of high speed receivers, it is important that the gain decisions be made quickly and accurately so that the overall data acquisition time is not adversely impacted.
Typical gain ranging methods employ either a separate analog measurement to determine required gain (e.g., such as a Received Signal Strength Indication (RSSI) or other scalar power detection scheme) or take a measurement using a high resolution data acquisition path, adjust the gain, and repeat until an acceptable signal is received. The former often does not necessarily have enough accuracy or range to properly select the gain and the latter can take too much time since a full measurement cycle must be repeated when a gain change is made.
FIG. 1 is an example of a typical receiver 100 with variable gain stages. The receiver 100 includes gain stages 102 and 104, each having a variable gain amplifier. Although FIG. 1 illustrates two gain stages, there can be several IF variable gain stages and several RF variable gain stages. In addition, there can be multiple downconversions, each with its own IF stage and each of those IF stages can have multiple variable gain stages. One common configuration uses several IF variable gain stages and no variable gain RF stages. In FIG. 1, the gain stages 102 and 104 adjust the gain of a RF input signal 105 and an IF signal 108 based on a desired signal to be provided to the input of an analog-to-digital (A/D) converter 110.
The downconverter 106 can include a local oscillator that converts the RF input signal 104 into an IF signal 108. The gain of the IF signal 108 can be adjusted by the gain stage 104 prior to receipt of the IF signal at the A/D converter 110. If the variable gain of amplifier 104 is set too low, then the receiver will have sub-optimal dynamic range and signal-to-noise ratios can be a problem. If the variable gain is set too high, there can be compression of the amplifiers or distortion of the A/D converter and signal. In both cases, the dynamic range of the receiver would be compromised.
The digital signal output of the A/D converter 110 is received by a digital signal processor (DSP), field programmable gate array (FPGA), or some other digital processing device as represented by processing block 114. Several techniques can be employed by the processing block 114 that have been employed in the past to decide whether to change the gain (termed ‘gain ranging’). In U.S. Pat. No. 3,636,463, entitled “Method of and Means for Gain-Ranging Amplification” (Ongkiehong), filed Dec. 12, 1969, the individual variable gain amplifiers are probed to determine when one amplifier is compressing and the preceding gain is reduced when this happens. In U.S. Pat. No. 5,861,831, entitled “Intermediate Frequency (IF) Sampling Clock-to-Clock Auto-Ranging Analog-to-Digital Converter (ADC) and Method” (Murden), filed Dec. 23, 1996, a peak detector is used prior to the A/D converter to determine what the amplitude of the signal is. By comparing this value to a list of predetermined gain-change values, it is known where to change the gain. In U.S. Pat. No. 3,699,325, entitled “Time-Shared Instantaneous Gain-Ranging Amplifier” (Montgomery), filed Oct. 9, 1969, a series of threshold detectors are used to accomplish the same task.
The above examples, however, typically use some analog hardware in conjunction with processing to determine the state of the signal and, hence, the required gain. In some cases, using primarily analog hardware to determine or measure the state of the IF signal to enable performing the gain ranging is not desired since the thresholds of the analog components used to check and reset gain levels may not be stable over time and temperature, or the decision process may be too slow for a high speed receiver. In addition, there may be synchronization issues between the analog circuits and the digital data acquisition process that require additional complexity to resolve. Such synchronization issues can also slow down the overall measurement process when a gain change is required. There may also be further complications if the incoming signal is spectrally complex in that the decision to change gain may be based on some attribute (e.g., total power) rather than the desired attribute (e.g., power at a specific frequency).
FIG. 2 illustrates an alternative to the analog hardware technique to determine whether gain ranging is required. In FIG. 2, the decision on whether to change the gain is based on the outcome of a Discrete Fourier Transform (DFT) that is determined primarily in the digital processor 114 of FIG. 1, rather than relying significantly on analog circuitry. There are typically stages of data reduction that can be performed in such a processor including, e.g., power measurements, peak value measurements and Fourier transforms, where the digital signal can be looked at to determine whether the gain needs adjusting. The configuration illustrated in FIG. 2 relies on a high resolution DFT as well as filtering performed in block 204. Once the DFT is performed, a decision can be made on whether gain ranging needs to occur. In gain determination block 206, a value of the DFT at a particular frequency is compared against a list of threshold values to determine if the gain needs to be adjusted. The list of threshold values can be in the form of a look up table. If the gain is to be changed, a control signal is provided from the processor back to the variable gain amplifier 102.
A disadvantage of the scenario of FIG. 2 is that in some cases the entire measurement cycle must be completed before it is known whether the gain has to be adjusted. In other words, if the gain does need to be adjusted, then the whole cycle must be repeated and the comparison performed again. Time delays during the gain ranging decision process itself may be important since the decision/gain change/settling process is serial in nature. These time penalties may be particularly troubling in a high speed receiver.